I use a Coolpix L22. 80 bucks, 12MP, built in macro. I leave the eyepiece on my microscope and just butt the camera lens up against the eyepiece. The small lens is very close in size to the microscope eyepiece lens size and so the image does fill the viewing screen completely, at least from top to bottom. I'm not sure why one would need an adapter unless their camera's objective lens was so large that you need to change the light path to accommodate the lens size and fill your screen. With the quality of small camera's now I would think it would be a LOT less expensive to get a small camera just for these sorts of pictures and call it good.
Holding the stone in a holder and hand holding the camera works great if you can get enough light into the stone. Here's a little crystal of something that was floating in the broth when the tourmaline surrounding it formed.
Canon PowerShot A570 just sitting on top of my microscope eye piece - who'd have thought it could be this easy!!!!!!! I was trying to overthink it and use my more expensive camera, when my little one works great! I just took this picture and didn't do anything but make it small enough to be able to upload. I didn't even hold it with tweezers or crop it or anything... who knew